Electroactive polymer fabrication

ABSTRACT

Pre-strained electroactive polymers are described that improve conversion from electrical to mechanical energy. When a voltage is applied to electrodes contacting a pre-strained polymer, the polymer deflects. This deflection may be used to do mechanical work. The pre-strain improves the mechanical response of an electroactive polymer. Also described herein are actuators that include an electroactive polymer and mechanical coupling to convert deflection of the polymer into mechanical work. Further described are compliant electrodes that conform to the shape of a polymer. Methods for fabricating electromechanical devices including one or more electroactive polymers are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) from co-pendingU.S. Provisional Patent Application No. 60/144,556 filed Jul. 20, 1999,naming R. E. Pelrine et al. as inventors, and titled “High-speedElectrically Actuated Polymers and Method of Use”, which is incorporatedby reference herein for all purposes; it also claims priority under 35U.S.C. §119(e) from co-pending U.S. Provisional Patent Application No.60/153,329 filed Sep. 10, 1999, naming R. E. Pelrine et al. asinventors, and titled “Electrostrictive Polymers As Microactuators”,which is incorporated by reference herein for all purposes; it alsoclaims priority under 35 U.S.C. §119(e) from co-pending U.S. ProvisionalPatent Application No. 60/161,325 filed Oct. 25, 1999, naming R. E.Pelrine et al. as inventors, and titled “Artificial MuscleMicroactuators”, which is incorporated by reference herein for allpurposes; it also claims priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/181,404 filed Feb.9, 2000, naming R. D. Kombluh et al. as inventors, and titled “FieldActuated Elastomeric Polymers”, which is incorporated by referenceherein for all purposes; it also claims priority under 35 U.S.C. §119(e)from co-pending U.S. Provisional Patent Application No. 60/187,809 filedMar. 8, 2000, naming R. E. Pelrine et al. as inventors, and titled“Polymer Actuators and Materials”, which is incorporated by referenceherein for all purposes; and it also claims priority under 35 U.S.C.§119(e) from co-pending U.S. Provisional Patent Application No.60/192,237 filed Mar. 27, 2000, naming R. D. Kombluh et al. asinventors, and titled “Polymer Actuators and Materials II”, which isincorporated by reference herein for all purposes; this application isalso a continuation in part of U.S. Patent Application entitled“Elastomeric Dielectric Polymer Film Sonic Actuator” naming R. E.Pelrine et al. as inventors, filed on Jul. 19, 1999 (U.S. applicationSer. No. 09/356,801 now U.S. Pat. No. 6,343,129), which is continuationfrom PCT/US98/02311 filed Feb. 2, 1998, which claims priority from U.S.Provisional application No. 60/037,400 filed Feb. 7, 1997, all of whichare incorporated by reference herein.

This invention is related to U.S. patent application Ser. No.09/620,025, filed on the same day as this patent application, naming R.Pelrine et al. as inventors. That application is incorporated herein byreference in its entirety and for all purposes.

This invention is also related to U.S. patent application Ser. No.09/619,846, filed on the same day as this patent application, naming R.Pelrine et al. as inventors. That application is incorporated herein byreference in its entirety and for all purposes.

This invention is also related to U.S. patent application Ser. No.09/619,848, filed on the same day as this patent application, naming R.Pelrine et al. as inventors. That application is incorporated herein byreference in its entirety and for all purposes.

This invention is also related to U.S. patent application Ser. No.09/619,843, filed on the same day as this patent application, naming R.Pelrine et al. as inventors. That application is incorporated herein byreference in its entirety and for all purposes.

This invention is also related to U.S. patent application Ser. No.09/619,847, filed on the same day as this patent application, naming Q.Pei et al. as inventors. That application is incorporated herein byreference in its entirety and for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to electroactive polymers thatconvert from electrical energy to mechanical energy. More particularly,the present invention relates to pre-strained polymers and their use inactuators and various applications. The present invention also relatesto compliant electrodes used to electrically communicate withelectroactive polymers and methods of fabricating pre-strained polymers.

In many applications, it is desirable to convert from electrical energyto mechanical energy. Exemplary applications requiring translation fromelectrical to mechanical energy include robotics, pumps, speakers,general automation, disk drives and prosthetic devices. Theseapplications include one or more actuators that convert electricalenergy into mechanical work—on a macroscopic or microscopic level.Common electric actuator technologies, such as electromagnetic motorsand solenoids, are not suitable for many of these applications, e.g.,when the required device size is small (e.g., micro or mesoscalemachines). These technologies are also not ideal when a large number ofdevices must be integrated into a single structure or under variousperformance conditions such as when high power density output isrequired at relatively low frequencies.

Several ‘smart materials’ have been used to convert between electricaland mechanical energy with limited success. These smart materialsinclude piezoelectric ceramics, shape memory alloys and magnetostrictivematerials. However, each smart material has a number of limitations thatprevent its broad usage. Certain piezoelectric ceramics, such as leadzirconium titanate (PZT), have been used to convert electrical tomechanical energy. While having suitable efficiency for a fewapplications, these piezoelectric ceramics are typically limited to astrain below about 1.6 percent and are often not suitable forapplications requiring greater strains than this. In addition, the highdensity of these materials often eliminates them from applicationsrequiring low weight. Irradiated polyvinylidene difluoride (PVDF) is anelectroactive polymer reported to have a strain of up to 4 percent whenconverting from electrical to mechanical energy. Similar to thepiezoelectric ceramics, the PVDF is often not suitable for applicationsrequiring strains greater than 4 percent. Shape memory alloys, such asnitinol, are capable of large strains and force outputs. These shapememory alloys have been limited from broad use by unacceptable energyefficiency, poor response time and prohibitive cost.

In addition to the performance limitations of piezoelectric ceramics andirradiated PVDF, their fabrication often presents a barrier toacceptability. Single crystal piezoelectric ceramics must be grown athigh temperatures coupled with a very slow cooling down process.Irradiated PVDF must be exposed to an electron beam for processing. Boththese processes are expensive and complex and may limit acceptability ofthese materials.

In view of the foregoing, alternative devices that convert fromelectrical to mechanical energy would be desirable.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to polymers that arepre-strained to improve conversion between electrical and mechanicalenergy. When a voltage is applied to electrodes contacting apre-strained polymer, the polymer deflects. This deflection may be usedto do mechanical work. The pre-strain improves the mechanical responseof an electroactive polymer relative to a non-strained polymer. Thepre-strain may vary in different directions of a polymer to varyresponse of the polymer to the applied voltage.

In another aspect, the present invention relates to actuators comprisingan electroactive polymer and mechanical coupling to convert deflectionof the polymer into mechanical output. Several actuators includemechanical coupling that improves the performance of an electroactivepolymer.

In yet another aspect, the present invention relates to compliantelectrodes that conform to the changing shape of a polymer. Many of theelectrodes are capable of maintaining electrical communication at thehigh deflections encountered with pre-strained polymers of the presentinvention. In some embodiments, electrode compliance may vary withdirection.

In another aspect, the present invention provides methods forfabricating electromechanical devices including one or moreelectroactive polymers. Pre-strain may be achieved by a number oftechniques such as mechanically stretching a polymer and fixing thepolymer to one or more solid members while it is stretched. Polymers ofthe present invention may be made by casting, dipping, spin coating,spraying or other known processes for fabrication of thin polymerlayers. In one embodiment, a pre-strained polymer comprises acommercially available polymer that is pre-strained during fabrication.

In another aspect, the present invention relates to a transducer fortranslating from electrical energy to mechanical energy. The transducerincludes at least two electrodes and a polymer arranged in a mannerwhich causes a portion of the polymer to deflect in response to a changein electric field. The polymer is elastically pre-strained.

In another aspect, the present invention relates to a transducer forconverting from electrical energy to mechanical energy. The transducercomprises at least two electrodes and a polymer arranged in a mannerwhich causes a portion of the polymer to deflect in response to a changein electric field provided by the at least two electrodes. The portionof the polymer deflects with a maximum linear strain between about 50percent and about 215 percent in response to the change in electricfield.

In yet another aspect, the present invention relates to an actuator forconverting electrical energy into displacement in a first direction. Theactuator comprises at least one transducer. Each transducer comprises atleast two electrodes and a polymer arranged in a manner which causes aportion of the polymer to deflect in response to a change in electricfield. The actuator also comprises a flexible frame coupled to the atleast one transducer, the frame providing mechanical assistance toimprove displacement in the first direction.

In another aspect, the present invention relates to an actuator forconverting electrical energy into mechanical energy. The actuatorcomprises a flexible member having fixed end and a free end, theflexible member comprising at least two electrodes and a pre-strainedpolymer arranged in a manner which causes a portion of the polymer todeflect in response to a change in electric field provided by the atleast two electrodes.

In another aspect, the present invention relates to an actuator forconverting electrical energy into displacement in a first direction. Theactuator comprises at least one transducer. Each transducer comprises atleast two electrodes and a polymer arranged in a manner which causes aportion of the polymer to deflect in response to a change in electricfield. The actuator also comprises at least one stiff member coupled tothe at least one transducer, the at least one stiff member substantiallypreventing displacement in a second direction.

In yet another aspect, the present invention relates to a diaphragmactuator for converting electrical energy into mechanical energy. Theactuator comprises at least one transducer. Each transducer comprises atleast two electrodes and a pre-strained polymer arranged in a mannerwhich causes a first portion of the polymer to deflect in response to achange in electric field. The actuator also comprises a frame attachedto a second portion of the polymer, the frame including at least onecircular hole, wherein the first portion deflects out of the plane ofthe at least one circular hole in response to the change in electricfield.

In another aspect, the present invention relates to an actuator forconverting electrical energy into mechanical energy, the actuatorcomprising a body having at least one degree of freedom between a firstbody portion and a second body portion, the body including at least onetransducer attached to the first portion and the second portion, eachtransducer comprising at least two electrodes and a pre-strained polymerarranged in a manner which causes a portion of the polymer to deflect inresponse to a change in electric field; the actuator also comprising afirst clamp attached to the first body portion and a second clampattached to the second body portion.

In yet another aspect, the present invention relates to an actuator forconverting electrical energy to mechanical energy, the actuatorcomprising a transducer, the transducer comprising a polymer arranged ina manner which causes a first portion of the polymer to deflect inresponse to a change in electric field, a first electrode pairconfigured to actuate a second portion of the polymer and a secondelectrode pair configured to actuate a third portion of the polymer, theactuator also comprising an output member coupled to a first portion ofthe polymer.

In another aspect, the present invention relates to an electrode for usewith an electroactive polymer. The electrode comprises a compliantportion in contact with the electroactive polymer, wherein the compliantportion is capable of deflection with a strain of at least about 50percent.

In yet another aspect, the present invention relates to an electrode foruse with an electroactive polymer. The electrode comprising a compliantportion in contact with the electroactive polymer, wherein the electrodecomprises an opacity which varies with deflection.

In yet another aspect, the present invention relates to an electrode foruse with an electroactive polymer. The electrode comprising a compliantportion in contact with the electroactive polymer, wherein the compliantportion comprises a textured surface.

In another aspect, the present invention relates to a method offabricating a transducer including a pre-strained polymer. The methodcomprises pre-straining an electroactive polymer to form thepre-strained polymer. The method also comprises fixing a portion of thepre-strained polymer to a solid member. The method additionallycomprises forming one or more electrodes on the pre-strained polymer.

In still another aspect, the present invention relates to a method offabricating a transducer comprising multiple pre-strained polymers. Themethod comprises pre-straining a first polymer to form a firstpre-strained polymer. The method also comprises forming one or moreelectrodes on the first pre-strained polymer. The method furthercomprises pre-straining a second polymer to form a second pre-strainedpolymer. The method additionally comprises forming one or moreelectrodes on the second pre-strained polymer. The method furthercomprises coupling the first pre-strained polymer to the secondpre-strained polymer.

These and other features and advantages of the present invention will bedescribed in the following description of the invention and associatedfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a top perspective view of a transducer beforeand after application of a voltage in accordance with one embodiment ofthe present invention.

FIG. 1C illustrates a textured surface for an electroactive polymerhaving a wavelike profile.

FIG. 1D illustrates an electroactive polymer including a texturedsurface having random texturing.

FIG. 1E illustrates a cross-sectional side view of a diaphragm actuatorincluding an electroactive polymer before application of a voltage inaccordance with one embodiment of the present invention.

FIG. 1F illustrates a cross-sectional view of the electroactive polymerdiaphragm of FIG. 1E after application of a voltage in accordance withone embodiment of the present invention.

FIGS. 2A and 2B illustrate a bow actuator before and after actuation inaccordance with a specific embodiment of the present invention.

FIG. 2C illustrates a bow actuator including additional components toimprove deflection in accordance with a specific embodiment of thepresent invention.

FIGS. 2D and 2E illustrate a linear motion actuator before and afteractuation in accordance with a specific embodiment of the presentinvention.

FIG. 2F illustrates a cross-sectional side view of an actuator includingmultiple polymer layers in accordance with one embodiment of the presentinvention.

FIG. 2G illustrates a stacked multilayer actuator as an example ofartificial muscle in accordance with one embodiment of the presentinvention.

FIG. 2H illustrates a linear actuator comprising an electroactivepolymer diaphragm in accordance with another embodiment of the presentinvention.

FIG. 2I illustrates an inchworm-type actuator including a rolledelectroactive polymer in accordance with one embodiment of the presentinvention.

FIG. 2J illustrates a stretched film actuator for providing deflectionin one direction in accordance with another embodiment of the presentinvention.

FIG. 2K illustrates a bending beam actuator in accordance with anotherembodiment of the present invention.

FIG. 2L illustrates the bending beam actuator of FIG. 2K with a 90degree bending angle.

FIG. 2M illustrates a bending beam actuator including two polymer layersin accordance with another embodiment of the present invention.

FIG. 3 illustrates a structured electrode that provides one-directionalcompliance according to a specific embodiment of the present invention.

FIG. 4 illustrates a pre-strained polymer comprising a structuredelectrode that is not directionally compliant according to a specificembodiment of the present invention.

FIG. 5 illustrates textured electrodes in accordance with one embodimentof the present invention.

FIG. 6 illustrates a two-stage cascaded pumping system including twodiaphragm actuator pumps in accordance with a specific embodiment of thepresent invention.

FIG. 7A illustrates a process flow for fabricating an electromechanicaldevice having at least one pre-strained polymer in accordance with oneembodiment of the present invention.

FIGS. 7B-F illustrate a process for fabricating an electromechanicaldevice having multiple polymer layers in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

1. Overview

Electroactive polymers deflect when actuated by electrical energy. Inone embodiment, an electroactive polymer refers to a polymer that actsas an insulating dielectric between two electrodes and may deflect uponapplication of a voltage difference between the two electrodes. In oneaspect, the present invention relates to polymers that are pre-strainedto improve conversion between electrical and mechanical energy. Thepre-strain improves the mechanical response of an electroactive polymerrelative to a non-strained electroactive polymer. The improvedmechanical response enables greater mechanical work for an electroactivepolymer, e.g., larger deflections and actuation pressures. For example,linear strains of at least about 200 percent and area strains of atleast about 300 percent are possible with pre-strained polymers of thepresent invention. The pre-strain may vary in different directions of apolymer. Combining directional variability of the pre-strain, differentways to constrain a polymer, scalability of electroactive polymers toboth micro and macro levels, and different polymer orientations (e.g.,rolling or stacking individual polymer layers) permits a broad range ofactuators that convert electrical energy into mechanical work. Theseactuators find use in a wide range of applications.

As the electroactive polymers of the present invention may deflect atlinear strains of at least about 200 percent, electrodes attached to thepolymers should also deflect without compromising mechanical orelectrical performance. Correspondingly, in another aspect, the presentinvention relates to compliant electrodes that conform to the shape ofan electroactive polymer they are attached to. The electrodes arecapable of maintaining electrical communication even at the highdeflections encountered with pre-strained polymers of the presentinvention. By way of example, strains of at least about 50 percent arecommon with electrodes of the present invention. In some embodiments,compliance provided by the electrodes may vary with direction.

As the pre-strained polymers are suitable for use in both the micro andmacro scales, in a wide variety of actuators and in a broad range ofapplications, fabrication processes used with the present invention varygreatly. In another aspect, the present invention provides methods forfabricating electromechanical devices including one or more pre-strainedpolymers. Pre-strain may be achieved by a number of techniques such asmechanically stretching an electroactive polymer and fixing the polymerto one or more solid members while it is stretched.

2. General Structure of Devices

FIGS. 1A and 1B illustrate a top perspective view of a transducer 100 inaccordance with one embodiment of the present invention. The transducer100 includes a polymer 102 for translating between electrical energy andmechanical energy. Top and bottom electrodes 104 and 106 are attached tothe electroactive polymer 102 on its top and bottom surfacesrespectively to provide a voltage difference across a portion of thepolymer 102. The polymer 102 deflects with a change in electric fieldprovided by the top and bottom electrodes 104 and 106. Deflection of thetransducer 100 in response to a change in electric field provided by theelectrodes 104 and 106 is referred to as actuation. As the polymer 102changes in size, the deflection may be used to produce mechanical work.

FIG. 1B illustrates a top perspective view of the transducer 100including deflection in response to a change in electric field.Generally speaking, deflection refers to any displacement, expansion,contraction, torsion, linear or area strain, or any other deformation ofa portion of the polymer 102. The change in electric field correspondingto the voltage difference produced by the electrodes 104 and 106produces mechanical pressure within the pre-strained polymer 102. Inthis case, the unlike electrical charges produced by the electrodes 104and 106 are attracted to each other and provide a compressive forcebetween the electrodes 104 and 106 and an expansion force on the polymer102 in planar directions 108 and 110, causing the polymer 102 tocompress between the electrodes 104 and 106 and stretch in the planardirections 108 and 110.

In some cases, the electrodes 104 and 106 cover a limited portion of thepolymer 102 relative to the total area of the polymer. This may done toprevent electrical breakdown around the edge of polymer 102 or achievecustomized deflections in certain portions of the polymer. As the termis used herein, an active region is defined as a portion of the polymermaterial 102 having sufficient electrostatic force to enable deflectionof the portion. As will be described below, a polymer of the presentinvention may have multiple active regions. Polymer 102 material outsidean active area may act as an external spring force on the active areaduring deflection. More specifically, material outside the active areamay resist active area deflection by its contraction or expansion.Removal of the voltage difference and the induced charge causes thereverse effects.

The electrodes 104 and 106 are compliant and change shape with thepolymer 102. The configuration of the polymer 102 and the electrodes 104and 106 provides for increasing polymer 102 response with deflection.More specifically, as the transducer 100 deflects, compression of thepolymer 102 brings the opposite charges of the electrodes 104 and 106closer and stretching of the polymer 102 separates similar charges ineach electrode. In one embodiment, one of the electrodes 104 and 106 isground.

Generally speaking, the transducer 100 continues to deflect untilmechanical forces balance the electrostatic forces driving thedeflection. The mechanical forces include elastic restoring forces ofthe polymer 102 material, the compliance of the electrodes 104 and 106,and any external resistance provided by a device and/or load coupled tothe transducer 100. The resultant deflection of the transducer 100 as aresult of the applied voltage may also depend on a number of otherfactors such as the polymer 102 dielectric constant and the polymer 102size.

Electroactive polymers in accordance with the present invention arecapable of deflection in any direction. After application of the voltagebetween the electrodes 104 and 106, the electroactive polymer 102increases in size in both planar directions 108 and 110. In some cases,the electroactive polymer 102 is incompressible, e.g. has asubstantially constant volume under stress. In this case, the polymer102 decreases in thickness as a result of the expansion in the planardirections 108 and 110. It should be noted that the present invention isnot limited to incompressible polymers and deflection of the polymer 102may not conform to such a simple relationship.

The electroactive polymer 102 is pre-strained. The pre-strain improvesconversion between electrical and mechanical energy. In one embodiment,pre-strain improves the dielectric strength of the polymer. For thetransducer 100, the pre-strain allows the electroactive polymer 102 todeflect more and provide greater mechanical work. Pre-strain of apolymer may be described in one or more directions as the change indimension in that direction after pre-straining relative to thedimension in that direction before pre-straining. The pre-strain maycomprise elastic deformation of the polymer 102 and be formed, forexample, by stretching the polymer in tension and fixing one or more ofthe edges while stretched. In one embodiment, the pre-strain is elastic.After actuation, an elastically pre-strained polymer could, inprinciple, be unfixed and return to its original state. The pre-strainmay be imposed at the boundaries using a rigid frame or may beimplemented locally for a portion of the polymer.

In one embodiment, pre-strain is applied uniformly over a portion of thepolymer 102 to produce an isotropic pre-strained polymer. By way ofexample, an acrylic elastomeric polymer may be stretched by 200-400percent in both planar directions. In another embodiment, pre-strain isapplied unequally in different directions for a portion of the polymer102 to produce an anisotropic pre-strained polymer. In this case, thepolymer 102 may deflect greater in one direction than another whenactuated. While not wishing to be bound by theory, it is believed thatpre-straining a polymer in one direction may increase the stiffness ofthe polymer in the pre-strain direction. Correspondingly, the polymer isrelatively stiffer in the high pre-strain direction and more compliantin the low pre-strain direction and, upon actuation, the majority ofdeflection occurs in the low pre-strain direction. In one embodiment,the transducer 100 enhances deflection in the direction 108 byexploiting large pre-strain in the perpendicular direction 110. By wayof example, an acrylic elastomeric polymer used as the transducer 100may be stretched by 100 percent in the direction 108 and by 500 percentin the perpendicular direction 110. Construction of the transducer 100and geometric edge constraints may also affect directional deflection aswill be described below with respect to actuators.

The quantity of pre-strain for a polymer may be based on theelectroactive polymer and the desired performance of the polymer in anactuator or application. For some polymers of the present invention,pre-strain in one or more directions may range from −100 percent to 600percent. By way of example, for a VHB acrylic elastomer having isotropicpre-strain, pre-strains of at least about 100 percent, and preferablybetween about 200-400 percent, may be used in each direction. In oneembodiment, the polymer is pre-strained by a factor in the range ofabout 1.5 times to 50 times the original area. For an anisotropicacrylic pre-strained to enhance actuation in a compliant direction,pre-strains between about 400-500 percent may be used in the stiffeneddirection and pre-strains between about 20-200 percent may be used inthe compliant direction. In some cases, pre-strain may be added in onedirection such that a negative pre-strain occurs in another direction,e.g. 600 percent in one direction coupled with −100 percent in anorthogonal direction. In these cases, the net change in area due to thepre-strain is typically positive.

Pre-strain may affect other properties of the polymer 102. Largepre-strains may change the elastic properties of the polymer and bringit into a stiffer regime with lower viscoelastic losses. For somepolymers, pre-strain increases the electrical breakdown strength of thepolymer 102, which allows for higher electric fields to be used withinthe polymer -permitting higher actuation pressures and higherdeflections.

Linear strain and area strain may be used to describe the deflection ofa pre-strained polymer. As the term is used herein, linear strain of apre-strained polymer refers to the deflection per unit length along aline of deflection relative to the unactuated state. Maximum linearstrains (tensile or compressive) of at least about 50 percent are commonfor pre-strained polymers of the present invention. Of course, a polymermay deflect with a strain less than the maximum, and the strain may beadjusted by adjusting the applied voltage. For some pre-strainedpolymers, maximum linear strains of at least about 100 percent arecommon. For polymers such as VHB 4910 as produced by 3M Corporation ofSt. Paul, Minn., maximum linear strains in the range of 40 to 215percent are common. Area strain of an electroactive polymer refers tothe change in planar area, e.g. the change in the plane defined bydirections 108 and 110 in FIGS. 1A and 1B, per unit area of the polymerupon actuation relative to the unactuated state. Maximum area strains ofat least about 100 percent are possible for pre-strained polymers of thepresent invention. For some pre-strained polymers, maximum area strainsin the range of 70 to 330 percent are common.

Generally, after the polymer is pre-strained, it may be fixed to one ormore objects. Each object may be suitably stiff to maintain the level ofpre-strain desired in the polymer. The polymer may be fixed to the oneor more objects according to any conventional method known in the artsuch as a chemical adhesive, an adhesive layer or material, mechanicalattachment, etc.

Transducers and pre-strained polymers of the present invention are notlimited to any particular geometry or linear deflection. For example,the polymer and electrodes may be formed into any geometry or shapeincluding tubes and rolls, stretched polymers attached between multiplerigid structures, stretched polymers attached across a frame of anygeometry—including curved or complex geometries, across a frame havingone or more joints, etc. Deflection of a transducer according to thepresent invention includes linear expansion and compression in one ormore directions, bending, axial deflection when the polymer is rolled,deflection out of a hole provided in a substrate, etc. Deflection of atransducer may be affected by how the polymer is constrained by a frameor rigid structures attached to the polymer. In one embodiment, aflexible material that is stiffer in elongation than the polymer isattached to one side of a transducer induces bending when the polymer isactuated. In another embodiment, a transducer that deflects radially outof the plane is referred to as a diaphragm. A diaphragm actuator will bedescribed in more detail with respect to FIGS. 1E and 1F.

Transducers (including methods of using them and methods of fabricatingthem) in accordance with the present invention are described in reportsavailable from the New Energy and Industrial Technology DevelopmentOrganization (NEDO) offices under the reference title “Annual ResearchProgress Report for R&D of Micromachine Technology (R&D of HighFunctional Maintenance System for Power Plant Facilities)” for 1999, the“Annual Research Progress Report for R&D of Micromachine Technology (R&Dof High Functional Maintenance System for Power Plant Facilities)” for1998, the “Annual Research Progress Report for R&D of MicromachineTechnology (R&D of High Functional Maintenance System for Power PlantFacilities)” for 1997, or the “Annual Research Progress Report for R&Dof Micromachine Technology (R&D of High Functional Maintenance Systemfor Power Plant Facilities)” for 1996, all of which are incorporatedherein for all purposes. NEDO has several offices in Japan in additionto other offices in the United Sates, Australia, France, Thailand andChina.

Electroactive polymers in accordance with one embodiment of the presentinvention may include a textured surface. FIG. 1C illustrates a texturedsurface 150 for an electroactive polymer 152 having a wavelike profile.The textured surface 150 allows the polymer 152 to deflect using bendingof surface waves 154. Bending of the surface waves 154 providesdirectional compliance in a direction 155 with less resistance than bulkstretching for a stiff electrode attached to the polymer 152 in thedirection 155. The textured surface 150 may be characterized by troughsand crests, for example, about 0.1 micrometer to 40 micrometers wide andabout 0.1 micrometers to 20 micrometers deep. In this case, the wavewidth and depth is substantially less than the thickness of the polymer.In a specific embodiment, the troughs and crests are approximately 10micrometers wide and six micrometers deep on a polymer layer with athickness of 200 micrometers.

In one embodiment, a thin layer of stiff material 156, such as anelectrode, is attached to the polymer 152 to provide the wavelikeprofile. During fabrication, the electroactive polymer is stretched morethan it can stretch when actuated, and the thin layer of stiff material156 is attached to the stretched polymer 152 surface. Subsequently, thepolymer 152 is relaxed and the structure buckles to provide the texturedsurface.

In general, a textured surface may comprise any non-uniform ornon-smooth surface topography that allows a polymer to deflect usingdeformation in the polymer surface. By way of example, FIG. 1Dillustrates an electroactive polymer 160 including a roughened surface161 having random texturing. The roughened surface 160 allows for planardeflection that is not directionally compliant. Advantageously,deformation in surface topography may allow deflection of a stiffelectrode with less resistance than bulk stretching or compression. Itshould be noted that deflection of a pre-strained polymer having atextured surface may comprise a combination of surface deformation andbulk stretching of the polymer.

Textured or non-uniform surfaces for the polymer may also allow the useof a barrier layer and/or electrodes that rely on deformation of thetextured surfaces. The electrodes may include metals that bend accordingto the geometry of the polymer surface. The barrier layer may be used toblock charge in the event of local electrical breakdown in thepre-strained polymer material.

Materials suitable for use as a pre-strained polymer with the presentinvention may include any substantially insulating polymer or rubberthat deforms in response to an electrostatic force or whose deformationresults in a change in electric field. One suitable material is NuSilCF19-2186 as provided by NuSil Technology of Carpenteria, Calif. Otherexemplary materials suitable for use as a pre-strained polymer include,any dielectric elastomeric polymer, silicone rubbers, fluoroelastomers,silicones such as Dow Corning HS3 as provided by Dow Corning ofWilmington, Del., fluorosilicones such as Dow Corning 730 as provided byDow Corning of Wilmington, Del., etc, and acrylic polymers such as anyacrylic in the 4900 VHB acrylic series as provided by 3M Corp. of St.Paul, Minn.

In many cases, materials used in accordance with the present inventionare commercially available polymers. The commercially available polymersmay include, for example, any commercially available silicone elastomer,polyurethane, PVDF copolymer and adhesive elastomer. Using commerciallyavailable materials provides cost-effective alternatives for transducersand associated devices of the present invention. The use of commerciallyavailable materials may simplify fabrication. In one embodiment, thecommercially available polymer is a commercially available acrylicelastomer comprising mixtures of aliphatic acrylate that are photocuredduring fabrication. The elasticity of the acrylic elastomer results froma combination of the branched aliphatic groups and crosslinking betweenthe acrylic polymer chains.

Materials used as a pre-strained polymer may be selected based on one ormore material properties such as a high electrical breakdownstrength, alow modulus of elasticity for large or small deformations, a highdielectric constant, etc. In one embodiment, the polymer is selectedsuch that is has an elastic modulus below 100 MPa. In anotherembodiment, the polymer is selected such that is has a maximum actuationpressure between about 0.05 MPa and about 10 MPa, and preferably betweenabout 0.3 MPa and about 3 MPa. In another embodiment, the polymer isselected such that is has a dielectric constant between about 2 andabout 20, and preferably between about 2.5 and about 12. For someapplications, an electroactive polymer is selected based on one or moreapplication demands such as a wide temperature and/or humidity range,repeatability, accuracy, low creep, reliability and endurance.

Suitable actuation voltages for pre-strained polymers of the presentinvention may vary based on the electroactive polymer material and itsproperties (e.g. the dielectric constant) as well as the dimensions ofthe polymer (e.g. the thickness between electrodes). By way of example,actuation electric fields for the polymer 102 in FIG. 1A may range inmagnitude from about 0 V/m to 440 MegaVolts/meter. Actuation voltages inthis range may produce a pressure in the range of about 0 Pa to about 10MPa. To achieve a transducer capable of higher forces, the thickness ofthe polymer may be increased. Alternatively, multiple polymer layers maybe implemented. Actuation voltages for a particular polymer may bereduced by increasing the dielectric constant, decreasing polymerthickness and decreasing the modulus of elasticity, for example.

Pre-strained polymers of the present invention may cover a wide range ofthicknesses. In one embodiment, polymer thickness may range betweenabout 1 micrometerand 2 millimeters. Typical thicknesses beforepre-strain include 50-225 micrometers for HS3, 25-75 micrometers forNuSil CF 19-2186, and 100-1000 micrometers for any of the 3M VHB 4900series acrylic polymers. Polymer thickness may be reduced by stretchingthe film in one or both planar directions. In many cases, pre-strainedpolymers of the present invention may be fabricated and implemented asthin films. Thicknesses suitable for these thin films may be below 50micrometers.

3. Actuators

The deflection of a pre-strained polymer can be used in a variety ofways to produce mechanical energy. Generally speaking, electroactivepolymers of the present invention may be implemented with a variety ofactuators—including conventional actuators retrofitted with apre-strained polymer and custom actuators specially designed for one ormore pre-strained polymers. Conventional actuators include extenders,bending beams, stacks, diaphragms, etc. Several different exemplarycustom actuators in accordance with the present invention will now bediscussed.

FIG. 1E illustrates a cross-sectional side view of a diaphragm actuator130 including a pre-strained polymer 131 before actuation in accordancewith one embodiment of the present invention. The pre-strained polymer131 is attached to a frame 132. The frame 132 includes a circular hole133 that allows deflection of the polymer 131 perpendicular to the areaof the circular hole 133. The diaphragm actuator 130 includes circularelectrodes 134 and 136 on either side of the polymer 131 to provide avoltage difference across a portion of the polymer 131.

In the voltage-off configuration of FIG. 1E, the polymer 131 isstretched and secured to the frame 132 with tension to achievepre-strain. Upon application of a suitable voltage to the electrodes 134and 136, the polymer film 131 expands away from the plane of the frame132 as illustrated in FIG. 1F. The electrodes 134 and 136 are compliantand change shape with the pre-strained polymer 131 as it deflects.

The diaphragm actuator 130 is capable of expansion in both directionsaway from the plane. In one embodiment, the bottom side 141 of thepolymer 131 includes a bias pressure that influences the expansion ofthe polymer film 131 to continually actuate upward in the direction ofarrows 143 (FIG. 1F). In another embodiment, a swelling agent such as asmall amount of silicone oil is applied to the bottom side 141 toinfluence the expansion of the polymer 131 in the direction of arrows143. The swelling agent causes slight permanent deflection in onedirection as determined during fabrication, e.g. by supplying a slightpressure on the bottom side 141 when the swelling agent is applied. Theswelling agent allows the diaphragm to continually actuate in a desireddirection without using a bias pressure.

The amount of expansion for the diaphragm actuator 130 will vary basedon a number of factors including the polymer 131 material, the appliedvoltage, the amount of pre-strain, any bias pressure, compliance of theelectrodes 134 and 136, etc. In one embodiment, the polymer 131 iscapable of deflections to a height 137 of at least about 50 percent ofthe hole diameter 139 and may take a hemispheric shape at largedeflections. In this case, an angle 147 formed between the polymer 131and the frame 132 may be less than 90 degrees.

As mentioned earlier, expansion in one direction of an electroactivepolymer may induce contractile stresses in a second direction such asdue to Poisson effects. This may reduce the mechanical output for atransducer that provides mechanical output in the second direction.Correspondingly, actuators of the present invention may be designed toconstrain a polymer in the non-output direction. In some cases,actuators may be designed to improve mechanical output using deflectionin the non-output direction.

An actuator which uses deflection in one planar direction to improvemechanical output in the other planar direction is a bow actuator. FIGS.2A and 2B illustrate a bow actuator 200 before and after actuation inaccordance with a specific embodiment of the present invention. The bowactuator 200 is a planar mechanism comprising a flexible frame 202 whichprovides mechanical assistance to improve mechanical output for apolymer 206 attached to the frame 202. The frame 202 includes six rigidmembers 204 connected at joints 205. The members 204 and joints 205provide mechanical assistance by coupling polymer deflection in a planardirection 208 into mechanical output in a perpendicular planar direction210. More specifically, the frame 202 is arranged such that a smalldeflection of the polymer 206 in the direction 208 improves displacementin the perpendicular planar direction 210. Attached to opposite (top andbottom) surfaces of the polymer 206 are electrodes 207 (bottom electrodeon bottom side of polymer 206 not shown) to provide a voltage differenceacross a portion of the polymer 206.

The polymer 206 is configured with different levels of pre-strain in itsorthogonal directions. More specifically, the electroactive polymer 206includes a high pre-strain in the planar direction 208, and little or nopre-strain in the perpendicular planar direction 210. This anisotropicpre-strain is arranged relative to the geometry of the frame 202. Morespecifically, upon actuation across electrodes 207 and 209, the polymercontracts in the high pre-strained direction 208. With the restrictedmotion of the frame 202 and the lever arm provided by the members 204,this contraction helps drive deflection in the perpendicular planardirection 210. Thus, even for a short deflection of the polymer 206 inthe high pre-strain direction 208, the frame 202 bows outward in thedirection 210. In this manner, a small contraction in the highpre-strain direction 210 becomes a larger expansion in the relativelylow pre-strain direction 208.

Using the anisotropic pre-strain and constraint provided by the frame202, the bow actuator 200 allows contraction in one direction to enhancemechanical deflection and electrical to mechanical conversion inanother. In other words, a load 211 (FIG. 2B) attached to the bowactuator 200 is coupled to deflection of the polymer 206 in twodirections direction 208 and 210. Thus, as a result of the differentialpre-strain of the polymer 206 and the geometry of the frame 202, the bowactuator 200 is able to provide a larger mechanical displacement than anelectroactive polymer alone for common electrical input.

The bow actuator 200 may be configured based on the polymer 206. By wayof example, the geometry of the frame 202 and dimensions of the polymer206 may be adapted based on the polymer 206 material. In a specificembodiment using HS3 silicone as the polymer 206, the polymer 206preferably has a ratio in directions 208 and 210 of 9:2 with pre-strainsabout 270 percent and −25 percent in the directions 208 and 210respectively. Using this arrangement, linear strains of at least about100 percent in direction 210 are possible.

The pre-strain in the polymer 206 and constraint provided by the frame202 may also allow the bow actuator 200 to utilize lower actuationvoltages for the pre-strained polymer 206 for a given deflection. As thebow actuator 200 has a lower effective modulus of elasticity in the lowpre-strained direction 210, the mechanical constraint provided by theframe 202 allows the bow actuator 200 to be actuated in the direction210 to a larger deflection with a lower voltage. In addition, the highpre-strain in the direction 208 increases the breakdown strength of thepolymer 206, permitting higher voltages and higher deflections for thebow actuator 200.

As mentioned earlier with respect FIG. 1A, when a polymer expands as aresult of electrostatic forces, it continues to expand until mechanicalforces balance the electrostatic pressure driving the expansion. Whenthe load 211 is attached to the bow actuator 200, mechanical effectsprovided by the load 211 will influence the force balance and deflectionof the polymer 206. For example, if the load 211 resists expansion ofthe bow actuator 200, then the polymer 206 may not expand as much as ifwere there no load.

In one embodiment, the bow actuator 200 may include additionalcomponents to provide mechanical assistance and enhance mechanicaloutput. By way of example, springs 220 as shown in FIG. 2C may beattached to the bow actuator 200 to enhance deflection in the direction210. The springs load the bow actuator 200 such that the spring forceexerted by the springs opposes resistance provided by an external load.In some cases, the springs 220 provide increasing assistance for bowactuator 200 deflection. In one embodiment, spring elements may be builtinto the joints 205 instead of the external springs 220 to enhancedeflection of the bow actuator 200. In addition, pre-strain may beincreased to enhance deflection. The load may also be coupled to therigid members 204 on top and bottom of the frame 202 rather than on therigid members of the side of the frame 202 (as shown in FIG. 2B). Sincethe top and bottom rigid members 204 contract towards each other whenvoltage is applied as shown in FIG. 2B, the bow actuator 200 provides anexemplary device contracts in the plane upon application of a voltagerather than expands.

Although the bow actuator 200 of FIGS. 2A-2C illustrates a specificexample of a custom actuator including a flexible frame and anelectroactive polymer, any frame geometry or mechanical assistance toimprove displacement of an electroactive polymer is suitable for usewith the present invention.

The shape and constraint of the polymer may affect deflection. An aspectratio for an electroactive polymer is defined as the ratio of its lengthto width. If the aspect ratio is high (e.g., an aspect ratio of at leastabout 4:1) and the polymer is constrained along its length by rigidmembers, than the combination may result in a substantially onedimensional deflection in the width direction.

FIGS. 2D and 2E illustrate a linear motion actuator 230 before and afteractuation in accordance with a specific embodiment of the presentinvention. The linear motion actuator 230 is a planar mechanismproviding mechanical output in one direction. The linear motion actuator230 comprises a polymer 231 having a length 233 substantially greaterthan its width 234 (e.g., an aspect ratio of at least about 4:1). Thepolymer 231 is attached on opposite sides to stiff members 232 of aframe along its length 233. The stiff members 232 have a greaterstiffness than the polymer 231. The geometric edge constraint providedby the stiff members 232 substantially prevents displacement in adirection 236 along the polymer length 233 and facilitates deflectionalmost exclusively in a direction 235. When the linear motion actuator230 is implemented with a polymer 231 having anisotropic pre-strain,such as a higher pre-strain in the direction 236 than in the direction235, then the polymer 231 is stiffer in the direction 236 than in thedirection 235 and large deflections in the direction 235 may result. Byway of example, such an arrangement may produce linear strains of atleast about 200 percent for acrylics having an anisotropic pre-strain.

A collection of electroactive polymers or actuators may be mechanicallylinked to form a larger actuator with a common output, e.g. force and/ordisplacement. By using a small electroactive polymer as a base unit in acollection, conversion of electric energy to mechanical energy may bescaled according to an application. By way of example, multiple linearmotion actuators 230 may be combined in series in the direction 235 toform an actuator having a cumulative deflection of all the linear motionactuators in the series. When transducing electric energy intomechanical energy, electroactive polymers—either individually ormechanically linked in a collection—may be referred to as ‘artificialmuscle’. For purposes herein, artificial muscle is defined as one ormore transducers and/or actuators having a single output force and/ordisplacement. Artificial muscle may be implemented on a micro or macrolevel and may comprise any one or more of the actuators describedherein.

FIG. 2F illustrates cross-sectional side view of a multilayer actuator240 as an example of artificial muscle in accordance with a specificembodiment of the present invention. The multilayer actuator 240includes four pre-strained polymers 241 arranged in parallel and eachattached to a rigid frame 242 such that they have the same deflection.Electrodes 243 and 244 are deposited on opposite surfaces of eachpolymer 241 and provide simultaneous electrostatic actuation to the fourpre-strained polymers 241. The multilayer actuator 240 providescumulative force output of the individual polymer layers 241.

In another embodiment, multiple electroactive polymer layers may be usedin place of one polymer to increase the force or pressure output of anactuator. For example, ten electroactive polymers may be layered toincrease the pressure output of the diaphragm actuator of FIG. 1E. FIG.2G illustrates such a stacked multilayer diaphragm actuator 245 asanother example of artificial muscle in accordance with one embodimentof the present invention. The stacked multilayer actuator 245 includesthree polymer layers 246 layered upon each other and may be attached byadhesive layers 247. Within the adhesive layers 247 are electrodes 248and 249 that provide actuation to polymer layers 246. A relatively rigidplate 250 is attached to the outermost polymer layer and patterned toinclude holes 251 that allow deflection for the stacked multilayerdiaphragm actuator 245. By combining the polymer layers 246, the stackedmultilayer actuator 245 provides cumulative force output of theindividual polymer layers 246.

In addition to the linear motion actuator 230 of FIGS. 2D and 2E,electroactive polymers of the present invention may be included in avariety of actuators that provide linear displacement. FIG. 2Hillustrates a linear actuator 255 comprising an electroactive polymerdiaphragm 256 in accordance with another embodiment of the presentinvention. In this case, an output shaft 257 is attached to a centralportion of the diaphragm 256 that deflects in a hole 258 of a frame 261.Upon actuation and removal of electrostatic energy, the output shaft 257translates as indicated by arrow 259. The linear actuator 255 may alsoinclude a compliant spring element 260 that helps position the outputshaft 257.

In another embodiment, pre-strained polymers of the present inventionmay be rolled or folded into linear transducers and actuators thatdeflect axially upon actuation. As fabrication of electroactive polymersis often simplest with fewer numbers of layers, rolled actuators providean efficient manner of squeezing large layers of polymer into a compactshape. Rolled or folded transducers and actuators may include one ormore layers of polymer rolled or folded to provide numerous layers ofpolymer adjacent to each other. Rolled or folded actuators areapplicable wherever linear actuators are used, such as robotic legs andfingers, high force grippers, and general-purpose linear actuators.

FIG. 2I illustrates an inchworm-type actuator 262 in accordance with aspecific embodiment of the present invention. The inchworm-type actuator262 includes two or more rolled pre-strained polymer layers withelectrodes 263 that deflect axially along its cylindrical axis. Theinchworm-type actuator 262 also includes electrostatic clamps 264 and265 for attaching and detaching to a metal surface 268. Theelectrostatic clamps 264 and 265 allow the total stroke for theinchworm-type actuator 262 to be increased compared to an actuatorwithout clamping. As the clamping force per unit weight for theelectrostatic clamps 264 and 265 is high, the force per unit weightadvantages of pre-strained polymers of the present invention arepreserved with the inchworm-type actuator 262. The electrostatic clamps264 and 265 are attached to the inchworm-type actuator at connectionregions 267. A body 266 of the inchworm-type actuator includes theconnection regions 267 and the polymer 263 and has a degree of freedomalong the axial direction of the rolled polymer 263 between theconnection regions 267. In one embodiment, the electrostatic clamps 264and 265 include an insulating adhesive 269 that prevents electricalshorting from the conductive electrostatic clamps 264 and 265 to themetal surface 268.

The inchworm-type actuator 262 moves upward in a six step process. Instep one, the inchworm-type actuator 262 is immobilized at itsrespective ends when both electrostatic clamps 264 and 265 are actuatedand the polymer 263 is relaxed. An electrostatic clamp is actuated byapplying a voltage difference between the clamp and the metal surface268. In step two, clamp 265 is released. Releasing one of the clamps 264and 265 allows its respective end of the inchworm-type actuator 262 tomove freely. In step three, the electroactive polymer 263 is actuatedand extends the inchworm-type actuator 262 upward. In step four, clamp265 is actuated and the inchworm-type actuator 262 is immobilized. Instep five, clamp 264 is released. In step six, the polymer 263 isrelaxed and the inchworm-type actuator 262 contracts. By cyclicallyrepeating steps one through six, the inchworm-type actuator 262 moves inthe upward direction. By switching clamps 264 and 265 in the above sixstep process, the inchworm-type actuator 262 moves in a reversedirection.

Although the inchworm-type actuator 262 has been described in terms ofactuation using a single electroactive polymer and two clamps, multiplesegment inchworm-type actuators using multiple electroactive polymersmay be implemented. Multiple segment inchworm-type actuators allow aninchworm-type actuator to increase in length without becoming thicker. Atwo-segment inchworm-type actuator would use two rolled polymers ratherthan one and three clamps rather than two. In general, an n-segmentinchworm-type actuator comprises n actuators between n+1 clamps.

FIG. 2J illustrates a stretched film actuator 270 for providing lineardeflection in accordance with another embodiment of the presentinvention. The stretched film actuator 270 includes a rigid frame 271having a hole 272. A pre-strained polymer 273 is attached in tension tothe frame 271 and spans the hole 272. A rigid bar 274 is attached to thecenter of the polymer 273 and provides external displacementcorresponding to deflection of the polymer 273. Compliant electrodepairs 275 and 276 are patterned on both top and bottom surfaces of thepolymer 273 on the left and right sides respectively of the rigid bar274. When the electrode pair 275 is actuated, a portion of the polymer273 between and in the vicinity of the top and bottom electrode pair 275expands relative to the rest of the polymer 273 and the existing tensionin the remainder of the polymer 273 pulls the rigid bar 274 to move tothe right. Conversely, when the electrode pair 276 is actuated, a secondportion of the polymer 273 affected by the electrode pair 276 expandsrelative to the rest of the polymer 273 and allows the rigid bar 274 tomove to the left. Alternating actuation of the electrodes 275 and 276provides an effectively larger total stroke 279 for the rigid bar 274.One variation of this actuator includes adding anisotropic pre-strain tothe polymer such that the polymer has high pre-strain (and stiffness) inthe direction perpendicular to the rigid bar displacement. Anothervariation is to eliminate one of the electrode pairs. For the benefit ofsimplifying the design, this variation reduces the stroke 279 for thestretched film actuator 270. In this case, the portion of the polymer nolonger used by the removed electrode now responds passively like arestoring spring.

FIG. 2K illustrates a bending beam actuator 280 in accordance withanother embodiment of the present invention. The bending beam actuator280 includes a polymer 281 fixed at one end by a rigid support 282 andattached to a flexible thin material 283 such as polyimide or mylarusing an adhesive layer, for example. The flexible thin material 283 hasa modulus of elasticity greater than the polymer 281. The difference inmodulus of elasticity for the top and bottom sides 286 and 287 of thebending beam actuator 280 causes the bending beam actuator 280 to bendupon actuation. Electrodes 284 and 285 are attached to the oppositesides of the polymer 281 to provide electrical energy. The bending beamactuator 280 includes a free end 288 having a single bending degree offreedom. Deflection of the free end 288 may be measured by thedifference in angle between the free end 288 and the end fixed by therigid support 282. FIG. 2L illustrates the bending beam actuator 280with a 90 degree bending angle.

The maximum bending angle for the bending beam actuator 280 will varywith a number of factors including the polymer material, the actuatorlength, the bending stiffness of the electrodes 284 and 285 and flexiblethin material 283, etc. For a bending beam actuator 280 comprising DowComing HS3 silicone, gold electrodes and an active area of 3.5 mm inlength, bending angles over 225 degrees are attainable. For the bendingbeam actuator 280, as the length of the active area increases, increasedbending angles are attainable. Correspondingly, by extending the activelength of the above mentioned bending beam actuator to 5 mm allows for abending angle approaching 360 degrees.

In one embodiment, one of the electrodes may act as the flexible thinmaterial 283. Any thin metal, such as gold, having a low bendingstiffness and a high tensile stiffness may be suitable for an electrodeacting as the flexible thin material 283. In another embodiment, abarrier layer is attached between one of the electrodes 284 and 285 andthe polymer 281 to minimize the effect of any localized breakdown in thepolymer. Breakdown may be defined as the point at which the polymercannot sustain the applied voltage. The barrier layer is typicallythinner than the polymer 281 and has a higher dielectric constant thanthe polymer 281 such that the voltage drop mainly occurs across thepolymer 281. It is often preferable that the barrier layer have a highdielectric breakdown strength.

FIG. 2M illustrates a bending beam actuator 290 in accordance withanother embodiment of the present invention. The bending beam actuator290 includes top and bottom pre-strained polymers 291 and 292 fixed atone end by a rigid support 296. Each of the polymers 291 and 292 may beindependently actuated. Independent actuation is achieved by separateelectrical control of top and bottom electrodes 293 and 294 attached tothe top and bottom electroactive polymers 291 and 292, respectively. Acommon electrode 295 is situated between the top and bottomelectroactive polymers 291 and 292 and attached to both. The commonelectrode 295 may be of sufficient stiffness to maintain the pre-strainon the polymer layers 291 and 292 while still permitting extension andbending.

Actuating the top electroactive polymer 291 using the top pair ofelectrodes 293 and 295 causes the bending beam actuator 290 to benddownward. Actuating the bottom polymer 292 using the bottom pair ofelectrodes 294 and 295 causes the bending beam actuator 290 to bendupward. Thus, independent use of the top and bottom electroactivepolymers 291 and 292 allows the bending beam actuator 290 to becontrolled along a radial direction 297. When both top and bottompolymers 291 and 292 are actuated simultaneously—and are ofsubstantially similar size and material—the bending beam actuator 290extends in length along the linear direction 298. Combining the abilityto control motion in the radial direction 297 and the linear direction298, the bending beam actuator 290 becomes a two-degree-of-freedomactuator. Correspondingly, independent actuation and control of the topand bottom polymers 291 and 292 allows a free end 299 of the bendingbeam actuator 290 to execute complex motions such as circular orelliptical paths.

4. Performance

A transducer in accordance with the present invention converts betweenelectrical energy and mechanical energy. Transducer performance may becharacterized in terms of the transducer by itself, the performance ofthe transducer in an actuator, or the performance of the transducer in aspecific application (e.g., a number of transducers implemented in amotor). Pre-straining electroactive polymers in accordance with thepresent invention provides substantial improvements in transducerperformance.

Characterizing the performance of a transducer by itself usually relatesto the material properties of the polymer and electrodes. Performance ofan electroactive polymer may be described independent of polymer size byparameters such as strain, energy density, actuation pressure, actuationpressure density and efficiency. It should be noted that the performancecharacterization of pre-strained polymers and their respectivetransducers described below may vary for different electroactivepolymers and electrodes.

Pre-strained polymers of the present invention may have an effectivemodulus in the range of about 0.1 to about 100 MPa. Actuation pressureis defined as the change in force within a pre-strained polymer per unitcross-sectional area between actuated and unactuated states. In somecases, pre-strained polymers of the present invention may have anactuation pressure in the range of about 0 to about 100 MPa, and morepreferably in the range 0.1 to 10 MPa. Specific elastic energydensity—defined as the energy of deformation of a unit mass of thematerial in the transition between actuated and unactuated states—mayalso be used to describe an electroactive polymer where weight isimportant. Pre-strained polymers of the present invention may have aspecific elastic energy density of over 3 J/g.

The performance of a pre-strained polymer may also be described,independent of polymer size, by efficiency. Electromechanical efficiencyis defined as the ratio of mechanical output energy to electrical inputenergy. Electromechanical efficiency greater than 80 percent isachievable with some pre-strained polymers of the present invention. Thetime for a pre-strained polymer to rise (or fall) to its maximum (orminimum) actuation pressure is referred to as its response time.Pre-strained polymer polymers in accordance with the present inventionmay accommodate a wide range of response times. Depending on the sizeand configuration of the polymer, response times may range from about0.01 milliseconds to 1 second, for example. A pre-strained polymerexcited at a high rate may also be characterized by an operationalfrequency. In one embodiment, maximum operational frequencies suitablefor use with the present invention may be in the range of about 100 Hzto 100 kHz. Operational frequencies in this range allow pre-strainedpolymers of the present invention to be used in various acousticapplications (e.g., speakers). In some embodiments, pre-strainedpolymers of the present invention may be operated at a resonantfrequency to improve mechanical output.

Performance of an actuator may be described by a performance parameterspecific to the actuator. By way of example, performance of an actuatorof a certain size and weight may be quantified by parameters such asstroke or displacement, force, actuator response time. Characterizingthe performance of a transducer in an application relates to how wellthe transducer is embodied in a particular application (e.g. inrobotics). Performance of a transducer in an application may bedescribed by a performance parameter specific to the application (e.g.,force/unit weight in robotic applications). Application specificparameters include stroke or displacement, force, actuator responsetime, frequency response, efficiency, etc. These parameters may dependon the size, mass and/or the design of the transducer and the particularapplication.

It should be noted that desirable material properties for anelectroactive polymer may vary with an actuator or application. Toproduce a large actuation pressures and large strain for an application,a pre-strained polymer may be implemented with one of a high dielectricstrength, a high dielectric constant, and a low modulus of elasticity.Additionally, a polymer may include one of a high-volume resistivity andlow mechanical damping for maximizing energy efficiency for anapplication.

5. Electrodes

As mentioned above, transducers of the present invention preferablyinclude one or more electrodes for actuating an electroactive polymer.Generally speaking, electrodes suitable for use with the presentinvention may be of any shape and material provided they are able tosupply or receive a suitable voltage, either constant or varying overtime, to or from an electroactive polymer. In one embodiment, theelectrodes adhere to a surface of the polymer. Electrodes adhering tothe polymer are preferably compliant and conform to the changing shapeof the polymer. The electrodes may be only applied to a portion of anelectroactive polymer and define an active area according to theirgeometry.

The compliant electrodes are capable of deflection in one or moredirections. Linear strain may be used to describe the deflection of acompliant electrode in one of these directions. As the term is usedherein, linear strain of a compliant electrode refers to the deflectionper unit length along a line of deflection. Maximum linear strains(tensile or compressive) of at least about 50 percent are possible forcompliant electrodes of the present invention. For some compliantelectrodes, maximum linear strains of at least about 100 percent arecommon. Of course, an electrode may deflect with a strain less than themaximum. In one embodiment, the compliant electrode is a ‘structuredelectrode’ that comprises one or more regions of high conductivity andone or more regions of low conductivity.

FIG. 3 illustrates a top surface view of a structured electrode 501 thatprovides one-directional compliance in accordance with one embodiment ofthe present invention. The structured electrode 501 includes metaltraces 502 patterned in parallel lines over a charge distribution layer503—both of which cover an active area of a polymer (not shown). Themetal traces 502 and charge distribution layer 503 are applied toopposite surfaces of the polymer. Thus, the cross section, from top tobottom, of a transducer including structured electrodes 501 on oppositesurfaces is: top metal traces, top charge distribution layer, polymer,bottom charge distribution layer, bottom metal traces. Metal traces 502on either surface of the polymer act as electrodes for electroactivepolymer material between them. In another embodiment, the bottomelectrode may be a compliant, uniform electrode. The charge distributionlayer 503 facilitates distribution of charge between metal traces 502.Together, the high conductivity metal traces 502 quickly conduct chargeacross the active area to the low conductivity charge distribution layer503 which distributes the charge uniformly across the surface of thepolymer between the traces 502. The charge distribution layer 503 iscompliant. As a result, the structured electrode 501 allows deflectionin a compliant direction 506 perpendicular to the parallel metal traces502.

Actuation for the entire polymer may be achieved by extending the lengthof the parallel metal traces 502 across the length of the polymer and byimplementing a suitable number of traces 502 across the polymer width.In one embodiment, the metal traces 502 are spaced at intervals in theorder of 400 micrometers and have a thickness of about 20 to 100nanometers. The width of the traces is typically much less than thespacing. To increase the overall speed of response for the structuredelectrode 501, the distance between metal traces 502 may be reduced. Themetal traces 502 may comprise gold, silver, aluminum and many othermetals and relatively rigid conductive materials. In one embodiment,metal traces on opposite surfaces of an electroactive polymer are offsetfrom one another to improve charge distribution through the polymerlayer and prevent direct metal-to-metal electrical breakdowns.

Deflection of the parallel metal traces 502 along their axis greaterthan the elastic allowance of the metal trace material may lead todamage of the metal traces 502. To prevent damage in this manner, thepolymer may be constrained by a rigid structure that prevents deflectionof the polymer and the metal traces 502 along their axis. The rigidmembers 232 of the linear motion actuator of FIGS. 2D and 2E aresuitable in this regard. In another embodiment, the metal traces 502 maybe undulated slightly on the surface of the polymer 500. Theseundulations add compliance to the traces 502 along their axis and allowdeflection in this direction.

In general, the charge distribution layer 503 has a conductance greaterthan the electroactive polymer but less than the metal traces. Thenon-stringent conductivity requirements of the charge distribution layer503 allow a wide variety of materials to be used. By way of example, thecharge distribution layer may comprise carbon black, fluoroelastomerwith colloidal silver, a water-based latex rubber emulsion with a smallpercentage in mass loading of sodium iodide, and polyurethane withtetrathiafulavalene/tetracyanoquinodimethane (TTF/TCNQ) charge transfercomplex. These materials are able to form thin uniform layers with evencoverage and have a surface conductivity sufficient to conduct thecharge between metal traces 502 before substantial charge leaks into thesurroundings. In one embodiment, material for the charge distributionlayer 503 is selected based on the RC time constant of the polymer. Byway of example, surface resistivity for the charge distribution layer503 suitable for the present invention may be in the range of 10⁶14 10¹¹ohms. It should also be noted that in some embodiments, a chargedistribution layer is not used and the metal traces 502 are patterneddirectly on the polymer. In this case, air or another chemical specieson the polymer surface may be sufficient to carry charge between thetraces. This effect may be enhanced by increasing the surfaceconductivity through surface treatments such as plasma etching or ionimplantation.

In another embodiment, multiple metal electrodes are situated on thesame side of a polymer and extend the width of the polymer. Theelectrodes provide compliance in the direction perpendicular to width.Two adjacent metal electrodes act as electrodes for polymer materialbetween them. The multiple metal electrodes alternate in this manner andalternating electrodes may be in electrical communication to providesynchronous activation of the polymer.

FIG. 4 illustrates a pre-strained polymer 510 underlying a structuredelectrode that is not directionally compliant according to a specificembodiment of the present invention. The structured electrode includesmetal traces 512 patterned directly on one surface of the electroactivepolymer 510 in evenly spaced parallel lines forming a ‘zig-zag’ pattern.Two metal traces 512 on opposite surfaces of the polymer act aselectrodes for the electroactive polymer 510 material between them. The‘zig-zag’ pattern of the metal traces 512 allows for expansion andcontraction of the polymer and the structure electrode in multipledirections 514 and 516.

Using an array of metal traces as described with respect to FIGS. 3 and4 permits the use of charge distribution layers having a lowerconductance. More specifically, as the spacing between metal tracesdecreases, the required conductance of the material between the tracesmay diminish. In this manner, it is possible to use materials that arenot normally considered conductive to be used as a charge distributionlayers. By way of example, polymers having a surface resistivity of 10¹⁰ohms may be used as an charge distribution layer in this manner. In aspecific embodiment, rubber was used as a charge distribution layer aspart of a structured electrode on a polymer layer having a thickness of25 micrometers and spacing between parallel metal traces of about 500micrometers. In addition to reducing the required conductance for acharge distribution layer, closely spaced metal traces also increase thespeed of actuation since the charge need only travel through the chargedistribution layer for a short distance between closely spaced metaltraces.

Although structured electrodes of the present invention have beendescribed in terms of two specific metal trace configurations;structured electrodes in accordance with the present invention may bepatterned in any suitable manner. As one skilled in the art willappreciate, various uniformly distributed metallic trace patterns mayprovide charge on the surface of a polymer while providing compliance inone or more directions. In some cases, a structured electrode may beattached to the surface of polymer in a non-uniform manner. As actuationof the polymer may be limited to an active region within suitableproximity of a pair of patterned metal traces, specialized active andnon-active regions for an electroactive polymer may be defined by custompatterning of the metal traces. These active and non-active regions maybe formed to custom geometries and high resolutions according toconventional metal trace deposition techniques. Extending this practiceacross the entire surface of an electroactive polymer, custom patternsfor structured electrodes comprising numerous custom geometry activeregions may result in specialized and non-uniform actuation of theelectroactive polymer according to the pattern of the structuredelectrodes.

Although the present invention has been discussed primarily in terms offlat electrodes, ‘textured electrodes’ comprising varying out of planedimensions may be used to provide a compliant electrode. FIG. 5illustrates exemplary textured electrodes 520 and 521 in accordance withone embodiment of the present invention. The textured electrodes 520 and521 are attached to opposite surfaces of an electroactive polymer 522such that deflection of the polymer 522 results in planar and non-planardeformation of the textured electrodes 520 and 521. The planar andnon-planar compliance of the electrodes 520 and 521 is provided by anundulating pattern which, upon planar and/or thickness deflection of thepolymer 522, provides directional compliance in a direction 526. Toprovide substantially uniform compliance for the textured electrodes 520and 521, the undulating pattern is implemented across the entire surfaceof the electroactive polymer in the direction 526. In one embodiment,the textured electrodes 520 and 521 are comprised of metal having athickness which allows bending without cracking of the metal to providecompliance. Typically, the textured electrode 520 is configured suchthat non-planar deflection of the electrodes 520 and 521 is much lessthan the thickness of the polymer 522 in order to provide asubstantially constant electric field to the polymer 522. Texturedelectrodes may provide compliance in more than one direction. In aspecific embodiment, a rough textured electrode provides compliance inorthogonal planar directions. The rough textured electrode may have atopography similar to the rough surface of FIG. 1D.

In one embodiment, compliant electrodes of the present inventioncomprise a conductive grease such as carbon grease or silver grease. Theconductive grease provides compliance in multiple directions. Particlesmay be added to increase the conductivity of the polymer. By way ofexample, carbon particles may be combined with a polymer binder such assilicone to produce a carbon grease that has low elasticity and highconductivity. Other materials may be blended into the conductive greaseto alter one or more material properties. Conductive greases inaccordance with the present invention are suitable for deflections of atleast about 100 percent strain.

Compliant electrodes of the present invention may also include colloidalsuspensions. Colloidal suspensions contain submicrometer sizedparticles, such as graphite, silver and gold, in a liquid vehicle.Generally speaking, any colloidal suspension having sufficient loadingof conductive particles may be used as an electrode in accordance withthe present invention. In a specific embodiment, a conductive greaseincluding colloidal sized conductive particles is mixed with aconductive silicone including colloidal sized conductive particles in asilicone binder to produce a colloidal suspension that cures to form aconductive semi-solid. An advantage of colloidal suspensions is thatthey may be patterned on the surface of a polymer by spraying, dipcoating and other techniques that allow for a thin uniform coating of aliquid. To facilitate adhesion between the polymer and an electrode, abinder may be added to the electrode. By way of example, a water-basedlatex rubber or silicone may be added as a binder to a colloidalsuspension including graphite.

In another embodiment, compliant electrodes are achieved using a highaspect ratio conductive material such as carbon fibrils and carbonnanotubes. These high aspect ratio carbon materials may form highsurface conductivities in thin layers. High aspect ratio carbonmaterials may impart high conductivity to the surface of the polymer atrelatively low electrode thicknesses due to the high interconnectivityof the high aspect ratio carbon materials. By way of example,thicknesses for electrodes made with common forms of carbon that are nothigh-aspect ratio may be in the range of 5-50 micrometers whilethicknesses for electrodes made with carbon fibril or carbon nanotubeelectrodes may be less than 2-4 micrometers. Area expansions well over100 percent in multiple directions are suitable with carbon fibril andcarbon nanotube electrodes on acrylic and other polymers. High aspectratio carbon materials may include the use of a polymer binder toincrease adhesion with the electroactive polymer layer. Advantageously,the use of polymer binder allows a specific binder to be selected basedon adhesion with a particular electroactive polymer layer and based onelastic and mechanical properties of the polymer.

In one embodiment, high-aspect-ratio carbon electrodes may be fabricatedthin enough such that the opacity of the electrodes may be variedaccording to polymer deflection. By way of example, the electrodes maybe made thin enough such that the electrode changes from opaque tosemitransparent upon planar expansion. This ability to manipulate theopacity of the electrode may allow transducers of the present inventionto be applied to a number of various optical applications as will bedescribed below.

In another embodiment, mixtures of ionically conductive materials may beused for the compliant electrodes. This may include, for example, waterbased polymer materials such as glycerol or salt in gelatin,iodine-doped natural rubbers and water-based emulsions to which organicsalts such as potassium iodide are added. For hydrophobic electroactivepolymers that may not adhere well to a water based electrode, thesurface of the polymer may be pretreated by plasma etching or with afine powder such as graphite or carbon black to increase adherence.

Materials used for the electrodes of the present invention may varygreatly. Suitable materials used in an electrode may include graphite,carbon black, colloidal suspensions, thin metals including silver andgold, silver filled and carbon filled gels. In a specific embodiment, anelectrode suitable for use with the present invention comprises 80percent carbon grease and 20 percent carbon black in a silicone rubberbinder such as Stockwell RTV60-CON as produced by Stockwell Rubber Co.Inc. of Philadelphia, Pa. The carbon grease is of the type such asCircuit Works 7200 as provided by ChemTronics Inc. of Kennesaw, Ga. Theconductive grease may also be mixed with an elastomer, such as siliconelastomer RTV 118 as produced by General Electric of Waterford, N.Y., toprovide a gel-like conductive grease.

It is understood that certain electrode materials may work well withparticular polymers and may not work as well for others. By way ofexample, carbon fibrils work well with acrylic elastomer polymers whilenot as well with silicone polymers. For most transducers, desirableproperties for the compliant electrode may include any one of a lowmodulus of elasticity, low mechanical damping, a low surfaceresistivity, uniform resistivity, chemical and environmental stability,chemical compatibility with the electroactive polymer, good adherence tothe electroactive polymer, and an ability to form smooth surfaces. Insome cases, it may be desirable for the electrode material to besuitable for precise patterning during fabrication. By way of example,the compliant electrode may be spray coated onto the polymer. In thiscase, material properties which benefit spray coating would bedesirable. In some cases, a transducer of the present invention mayimplement two different types of electrodes. By way of example, adiaphragm actuator of the present invention may have a structuredelectrode attached to its top surface and a high aspect ratio carbonmaterial deposited on the bottom side.

Electronic drivers are connected to the electrodes. The voltage providedto electroactive polymer will depend upon specifics of an application.In one embodiment, a transducer of the present invention is drivenelectrically by modulating an applied voltage about a DC bias voltage.Modulation about a bias voltage allows for improved sensitivity andlinearity of the transducer to the applied voltage. By way of example, atransducer used in an audio application may be driven by a signal of upto 200 to 1000 volts peak to peak on top of a bias voltage ranging fromabout 750 to 2000 volts DC.

6. Applications

As the present invention includes transducers that may be implemented inboth the micro and macro scales, and with a wide variety of actuatordesigns, the present invention finds use in a broad range ofapplications where electrical energy is converted into mechanicalenergy. Provided below are several exemplary applications for some ofthe actuators described above. Broadly speaking, the transducers andactuators of the present invention may find use in any applicationrequiring conversion from mechanical to electrical energy.

As mentioned before, electroactive polymers, either individually ormechanically linked in a collection, may be referred to as artificialmuscle. The term artificial muscle in itself implies that theseactuators are well-suited for application to biologically inspiredrobots or biomedical applications where the duplication of muscle,mammalian or other, is desired. By way of example, applications such asprosthetic limbs, exoskeletons, and artificial hearts may benefit frompre-strained polymers of the present invention. The size scalability ofelectroactive polymers and the ability to use any number of transducersor polymer actuators in a collection allow artificial muscle inaccordance with the present invention to be used in a range inapplications greater than their biological counterparts. As transducersand actuators of the present invention have a performance range faroutside their biological counterparts, the present invention is notlimited to artificial muscle having a performance corresponding to realmuscle, and may indeed include applications requiring performanceoutside that of real muscle.

In one example of artificial muscle, a collection of linear motionactuators comprises two or more layers of pre-strained polymersandwiched together and attached to two rigid plates at opposite edgesof each polymer. Electrodes are sealed into the center between each ofthe polymer layers. All of the linear motion actuators in the collectionmay take advantage of geometric constraints provided by the rigid platesand anisotropic pre-strain to restrict deformation of the polymer in theactuated direction. An advantage of the layered construction is that asmany electroactive polymer layers as required may be stacked in parallelin order to produce the desired force. Further, the stroke of thislinear motion actuator configuration may be increased by adding similarlinear motion actuators in series.

In the micro domain, the pre-strained polymers may range in thicknessfrom several micrometers to several millimeters and preferably fromseveral micrometers to hundreds of micrometers. Micro pre-strainedpolymers are well-suited for applications such as inkjets, actuatedvalves, micropumps, inchworm-type actuators, pointing mirrors, soundgenerators, microclamps, and micro robotic applications. Micro roboticapplications may include micro robot legs, grippers, pointer actuatorsfor CCD cameras, wire feeders for micro welding and repair, clampingactuators to hold rigid positions, and ultrasonic actuators to transmitdata over measured distances. In another application, a diaphragmactuator may be implemented in an array of similar electroactive polymerdiaphragms in a planar configuration on a single surface. By way ofexample, an array may include sixty-two diaphragms with the diameter of150 micrometers each arranged in a planar configuration. In oneembodiment, the array of diaphragm actuators may be formed on a siliconwafer. Diaphragm actuator arrays produced in this manner may include,for example, from 5 to 10,000 or more diaphragms each having a diameterin the range of 60 to 150 micrometers. The array may be placed upon gridplates having suitably spaced holes for each diaphragm.

In the macro domain, each of the actuators described above may be wellsuited to its own set of applications. For example, the inchworm-typeactuator of FIG. 2I is suitable for use with small robots capable ofnavigating through pipes less than 2 cm in diameter. Other actuators arewell-suited, for example, with applications such as robotics, solenoids,sound generators, linear actuators, aerospace actuators, and generalautomation.

In another embodiment, a transducer of the present invention is used asan optical modulation device or an optical switch. The transducerincludes an electrode whose opacity varies with deflection. Atransparent or substantially translucent pre-strained polymer isattached to the opacity varying electrode and deflection of the polymeris used to modulate opacity of device. In the case of an optical switch,the opacity varying transducer interrupts a light source communicatingwith a light sensor. Thus, deflection of the transparent polymer causesthe opacity varying electrode to deflect and affect the light sensor. Ina specific embodiment, the opacity varying electrode includes carbonfibrils or carbon nanotubes that become less opaque as electrode areaincreases and the area fibril density decreases. In another specificembodiment, an optical modulation device comprised of an electroactivepolymer and an opacity varying electrode may be designed to preciselymodulate the amount of light transmitted through the device.

Diaphragm actuators may be used as pumps, valves, etc. In oneembodiment, a diaphragm actuator having a pre-strained polymer issuitable for use as a pump. Pumping action is created by repeatedlyactuating the polymer. Electroactive polymer pumps in accordance withthe present invention may be implemented both in micro and macro scales.By way of example, the diaphragm may be used as a pump having a diameterin the range of about 150 micrometers to about 2 centimeters. Thesepumps may include polymer strains over 100 percent and may producepressures of 20 kPa or more.

FIG. 6 illustrates a two-stage cascaded pumping system includingdiaphragm pumps 540 and 542 in accordance with a specific embodiment ofthe present invention. The diaphragm pumps 540 and 542 includepre-strained polymers 544 and 546 attached to frames 545 and 547. Thepolymers 544 and 546 deflect within holes 548 and 550 in the frames 545and 547 respectively in a direction perpendicular to the plane of theholes 548 and 550. The frames 545 and 547 along with the polymers 544and 546 define cavities 551 and 552. The pump 540 includes a plunger 553having a spring 560 for providing a bias to the diaphragm 544 towardsthe cavity 551.

A one-way valve 555 permits inlet of a fluid or gas into the cavity 551.A one-way valve 556 permits outlet of the fluid or gas out of the cavity551 into the cavity 552. In addition, a one-way valve 558 permits exitof the fluid or gas from the cavity 552. Upon actuation of the polymers544 and 546, the polymers deflect in turn to change the pressure withinthe cavities 551 and 552 respectively, thereby moving fluid or gas fromthe one-way valve 555 to the cavity 551, out the valve 556, into thecavity 552, and out the valve 558.

In the cascaded two-stage pumping system of FIG. 6, the diaphragm pump542 does not include a bias since the pressurized output from thediaphragm pump 540 biases the pump 542. In one embodiment, only thefirst pump in a cascaded series of diaphragm pumps uses a biaspressure—or any other mechanism for self priming. In some embodiments,diaphragm pumps provided in an array may include voltages provided byelectronic timing to increase pumping efficiency. In the embodimentshown in FIG. 6, polymers 544 and 546 are actuated simultaneously forbest performance. For other embodiments which may involve more diaphragmpumps in the cascade, the electronic timing for the different actuatorsis ideally set so that one pump contracts in cavity volume while thenext pump in the series (as determined by the one-way valves) expands.In a specific embodiment, the diaphragm pump 540 supplies gas at a rateof 40 ml/min and a pressure about 1 kPa while the diaphragm pump 542supplies gas at substantially the same flow rate but increases thepressure to 2.5 kPa.

Bending beam actuators, such as those described with respect to FIGS.2K-2M, may be used in a variety of commercial and aerospace devices andapplications such as fans, electrical switches and relays, and lightscanners—on the micro and macro level. For bending beam actuators usedas light scanners, a reflective surface such as aluminized mylar may bebonded to the free end of a bending beam actuator. More specifically,light is reflected when the bending beam is actuated and light passeswhen the bending beam is at rest. The reflector may then be used toreflect incoming light and form a scanned beam to form an arc or lineaccording to the deflection of the actuator. Arrays of bending beamactuators may also be used for flat-panel displays, to control airflowover a surface, for low profile speakers and vibration suppressors, as“smart furs” for controlling heat transfer and/or light absorption on asurface, and may act as cilia in a coordinated manner to manipulateobjects.

Polymers and polymer films that are rolled into a tubular or multilayercylinder actuator may be implemented as a piston that expands axiallyupon actuation. Such an actuator is analogous to a hydraulic orpneumatic piston, and may be implemented in any device or applicationthat uses these traditional forms of linear deflection.

An electroactive polymer actuator may also operate at high speeds for avariety of applications including sound generators and acousticspeakers, inkjet printers, fast MEMS switches etc. In a specificembodiment, an electroactive polymer diaphragm is used as a lightscanner. More specifically, a mirror may be placed on a flexure thatpushes down on a 5 mm diameter electroactive polymer diaphragm toprovide a mirrored flexure. Good scanning of images at a scanning anglefrom about 10 to 30 degrees may be accomplished with voltages in therange of about 190 to 300 volts and frequencies in the range of about 30to 300 Hz. Much larger scanning angles, up to 90 degrees for example,may also be accommodated using voltages in the range of 400 to 500 V. Inaddition, higher frequencies may be used with a stiffer mirroredflexure.

7. Fabrication

As the pre-strained polymers may be implemented both in the micro andmacro scales, in a wide variety of actuator designs, with a wide rangeof materials, and in a broad range of applications, fabricationprocesses used with the present invention may vary greatly. In oneaspect, the present invention provides methods for fabricatingelectromechanical devices including one or more pre-strained polymers.

FIG. 7A illustrates a process flow 600 for fabricating anelectromechanical device having at least one electroactive polymer layerin accordance with one embodiment of the present invention. Processes inaccordance with the present invention may include up to severaladditional steps not described or illustrated here in order not toobscure the present invention. In some cases, fabrication processes ofthe present invention may include conventional materials and techniquessuch as commercially available polymers and techniques used infabrication of microelectronics and electronics technologies. Forexample, micro diaphragm actuators may be produced in situ on siliconusing conventional techniques to form the holes and apply the polymerand electrodes.

The process flow 600 begins by receiving or fabricating a polymer (602).The polymer may be received or fabricated according to several methods.In one embodiment, the polymer is a commercially available product suchas a commercially available acrylic elastomer film. In anotherembodiment, the polymer is a film produced by one of casting, dipping,spin coating or spraying. In one embodiment, the polymer is producedwhile minimizing variations in thickness or any other defects that maycompromise the maximize electric field that can be applied across thepolymer and thus compromise performance.

Spin coating typically involves applying a polymer mixture on a rigidsubstrate and spinning to a desired thickness. The polymer mixture mayinclude the polymer, a curing agent and a volatile dispersant orsolvent. The amount of dispersant, the volatility of the dispersant, andthe spin speed may be altered to produce a desired polymer. By way ofexample, polyurethane films may be spin coated in a solution ofpolyurethane and tetrahydrofuran (THF) or cyclohexanone. In the case ofsilicon substrates, the polymer may be spin coated on an aluminizedplastic or a silicon carbide. The aluminum and silicon carbide form asacrificial layer that is subsequently removed by a suitable etchant.Films in the range of one micrometer thick may been produced by spincoating in this manner. Spin coating of polymer films, such as silicone,may be done on a smooth non-sticking plastic substrate, such aspolymethyl methacrylate or teflon. The polymer film may then be releasedby mechanically peeling or with the assistance of alcohol or othersuitable release agent. Spin coating is also suitable for producingthicker polymers in the range of 10-750 micrometers. In some cases, thepolymer mixture may be centrifuged prior to spin coating to removeunwanted materials such as fillers, particulates, impurities andpigments used in commercial polymers. To increase centrifuge efficacy orto improve thickness consistency, a polymer may be diluted in a solventto lower its viscosity; e.g. silicone may be disbursed in naptha.

The polymer may then be pre-strained in one or more directions (604). Inone embodiment, pre-strain is achieved by mechanically stretching apolymer in or more directions and fixing it to one or more solid members(e.g, rigid plates) while strained. Another technique for maintainingpre-strain includes the use of one or more stiffeners. The stiffenersare long rigid structures placed on a polymer while it is in apre-strained state, e.g. while it is stretched. The stiffeners maintainthe pre-strain along their axis. The stiffeners may be arranged inparallel or other configurations to achieve directional compliance ofthe transducer. It should be noted that the increased stiffness alongthe stiffener axis comprises the increased stiffness provided by thestiffener material as well as the increased stiffness of the polymer inthe pre-strain direction.

Surfaces on the pre-strained polymer may be textured. In one embodimentto provide texturing, a polymer is stretched more than it can stretchwhen actuated, and a thin layer of stiff material is deposited on thestretched polymer surface. For example, the stiff material may be apolymer that is cured while the electroactive polymer is stretched.After curing, the electroactive polymer is relaxed and the structurebuckles to provide the textured surface. The thickness of the stiffmaterial may be altered to provide texturing on any scale, includingsubmicrometer levels. In another embodiment, textured surfaces areproduced by reactive ion etching (RIE). By way of example, RIE may beperformed on a pre-strained polymer comprising silicon with an RIE gascomprising 90 percent carbon tetrafluoride and 10 percent oxygen to forma surface with wave troughs and crests 4 to 5 micrometers in depth.

One or more electrodes are then formed on the polymer (606). For thesilicone polymer altered by RIE mentioned above, a thin layer of goldmay be sputter deposited on the RIE textured surface to provide atextured electrode. In another embodiment, one or more graphiteelectrodes may be patterned and deposited using a stencil. Electrodescomprising conductive greases mixed with a conductive silicone may befabricated by dissolving the conductive grease and the uncuredconductive silicone in a solvent. The solution may then be sprayed onthe electroactive polymer material and may include a mask or stencil toachieve a particular pattern.

The metal traces of the structured electrodes of FIGS. 3 and 4 may bepatterned photolithographically on top of the polymer or chargedistribution layer. By way of example, a layer of gold is sputterdeposited before depositing a photoresist over the gold. The photoresistand gold may be patterned according to conventional photolithographictechniques, e.g. using a mask followed by one or more rinses to removethe photoresist. A charge distribution layer added between the polymerand the metal traces may be deposited by spin coating, for example.

In a specific embodiment, a structured electrode is formed on a polymerby sputter depositing gold for about 2 to 3 minutes (according to adesired thickness) at about 150 angstroms per minute. HPR 506photoresist as provided by Arch Chemicals, of Norwalk, Conn. is thenspin coated on the gold at about 500 to 1500 rpm for about 20 to 30seconds and then baked at about 90 degrees Celsius. A mask is thenapplied before exposing the photoresist to UV light and development toremove unmasked portions of the photoresist. The gold is then etchedaway and the film is rinsed. The remaining photoresist is removed byexposure to UV light, development and rinsing. The gold traces may thenbe stretched to enhance strain tolerance.

Textured electrodes of the present invention may also be patternedphotolithographically. In this case, a photoresist is deposited on apre-strained polymer and patterned using a mask. Plasma etching mayremove portions of the electroactive polymer not protected by the maskin a desired pattern. The mask may be subsequently removed by a suitablewet etch. The active surfaces of the polymer may then be covered withthe thin layer of gold deposited by sputtering, for example.

The transducer, comprising the one or more polymer layers andelectrodes, is then packaged according to an application (608).Packaging may also include assembly of multiple transducers mechanicallylinked or stacked as multiple layers. In addition, mechanical andelectrical connections to the transducers may be formed according to anapplication.

The present invention also provides alternative methods for fabricatingelectromechanical devices including multiple layers of pre-strainedpolymer. In one embodiment, a process for fabricating electromechanicaldevices begins by obtaining or fabricating a polymer layer. The polymeris then stretched to the desired pre-strain and attached to a firstrigid frame. Next, electrodes are deposited onto both sides of thepolymer so as to define active areas and establish electricalconnections. The electrodes may be patterned by a variety of well-knowntechniques such as spray coating through a mask. If desired, a secondpolymer layer is then stretched on a second frame. Electrodes are thenpatterned on this second polymer layer. The second polymer layer is thencoupled to the first layer by stacking their respective frames. Layersof suitable compliant adhesives may be used to bond the two layers andelectrodes, if needed. The size of the frames is chosen so as not tointerfere with the polymer layers making intimate contact. Ifinterference is present, then it may be desirable to remove the secondframe, e.g., by cutting away the polymer layer around the periphery ofthe first frame. If desired, a third layer of polymer with electrodesmay be added in a manner similar to how the second layer was added tothe first. This procedure may be continued until a desired number oflayers is reached.

Rigid frames, rigid members or other electrical and mechanicalconnectors are then attached to the polymer layers, e.g., by gluing. Ifdesired, the polymer may then be removed from the first frame. In somecases, the first frame may serve as a structural part of the finalactuator or actuators. For example, the first frame may be an array ofholes to produce an array of diaphragm actuators.

FIGS. 7B-F illustrate a second process for fabricating anelectromechanical device 640 having multiple layers of electroactivepolymer in accordance with another embodiment of the present invention.Processes in accordance with the present invention may include up toseveral additional steps not described or illustrated here in order notto obscure the present invention. The process begins by producing apre-strained polymer 622 on a suitable rigid substrate 624, e.g. by spincoating a polymer on a polymethyl methacrylate (PMMA) disk, stretchingthe polymer (FIG. 7B) and then attaching it to rigid substrate 624.After the polymer 622 is cured, electrodes 625 are patterned on theexposed side 626 of the polymer 622. A solid member 627 such as aflexible film including one of polyimide, mylar or acetate film is thendeposited onto the electroactive polymer 622 (FIG. 7C) with a suitableadhesive 628.

The rigid substrate 624 is then released from the electroactive polymer622 (FIG. 7D). A releasing agent such as isopropyl alcohol may be usedto facilitate the release. Electrodes 629 are then patterned on thepreviously unexposed side of the polymer 622. The assembly is thenbonded to another electroactive polymer layer 630 attached to a rigidsubstrate 631 (FIG. 7E). Polymer layers 622 and 630 may be bonded by anadhesive layer 632 comprising GE RTV 118 silicone, for example. Therigid substrate 631 is then released from the polymer 630 and electrodes633 are patterned on the available side 634 of the polymer 630. Ifadditional polymer layers are desired, the steps of adding a polymerlayer, removing the rigid substrate, and adding electrodes may berepeated to produce as many polymer layers as desired. Polymer layer 635has been added in this manner. To facilitate electrical communication toelectrodes in the inner layers of the device 640, a metal pin may bepushed through the structure to make contact with electrodes in eachlayer.

The solid member 627 may then be patterned or removed as needed toprovide the frame or mechanical connections required by the specificactuator type. In one embodiment, diaphragm actuators may be formed bypatterning solid member 627 to form holes 636 which provide activeregions for the electromechanical device 640 using a suitable mask oretch technique (FIG. 7F). In another embodiment, if the active area isnot large and electrodes may be added to the active regions of thepolymers without damage, the solid member 627 may be patterned with theholes 636 prior to attachment to the polymer 622.

For the process of FIGS. 7B-F, the rigid substrate 624 is typicallyreleased from the electroactive polymer 622 by peeling the flexibleelectroactive polymer. Peeling is well-suited for fabricating devicescomprising electroactive polymers with a substantially flat profile. Inanother embodiment, sacrificial layers may be used between the polymeror electrodes and the rigid substrate to facilitate release. Thesacrificial layers allow the polymer, electrodes and attached assemblyto be released from a rigid substrate by etching away the sacrificiallayer. Metals comprising aluminum and silver are suitable for use as thesacrificial layers, for example. The use of metals allows thesacrificial layers to be etched away by liquids that do not affect thepolymer layers. Metal sacrificial layers may also be easily patternedwith various masking techniques to provide frames, connectors for otherstructural components for the electromechanical device 640. Thesacrificial layers may also be used to fabricate devices comprisingtransducers with non flat profiles, e.g. using rigid substrates shapedas tubes. For geometrically complex transducers, sacrificial layers maybe used in combination with dip coating to provide the complex geometry.

Although fabrication of pre-strained polymers has been briefly describedwith respect to a few specific examples, fabrication processes andtechniques of the present invention may vary accordingly for any theactuators or applications described above. For example, the process forfabricating a diaphragm actuator in accordance with a specificembodiment may include spin coating a polymer on a substrate before astructured electrode is fabricated on the polymer. The polymer is thenstretched and rigid frames including one or more holes sized for theactive area of each diaphragm actuator are bonded to the pre-strainedpolymer, including any overlap portions of the structured electrode. Inanother embodiment, holes are etched into the substrate instead of usinga separate rigid frame, e.g. when the substrate is comprised of silicon.The substrate is then released from the polymer and an electrode isattached to the bottom side of the polymer.

8. CONCLUSION

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents thatfall within the scope of this invention which have been omitted forbrevity's sake. By way of example, although the present invention hasbeen described in terms of several numerous applied material electrodes,the present invention is not limited to these materials and in somecases may include air as an electrode. It is therefore intended that thescope of the invention should be determined with reference to theappended claims.

What is claimed is:
 1. A method of fabricating a transducer comprising apre-strained polymer and one or more electrodes, the method comprising:pre-straining a first portion of a polymer to form the pre-strainedpolymer, wherein the polymer has an elastic modulus below 100 MPa;fixing a second portion of the pre-strained polymer to a solid member;and forming the one or more electrodes on the pre-strained polymer. 2.The method of claim 1 wherein the pre-strained polymer is elasticallypre-strained.
 3. The method of claim 1 wherein the polymer is one of acommercially available silicone elastomer, polyurethane, PVDF copolymeror adhesive elastomer.
 4. The method of claim 1 further comprising spincoating a polymer mixture to produce the polymer.
 5. The method of claim4 further comprising releasing the polymer from a rigid substrate usedin the spin coating.
 6. The method of claim 1 further comprisingtexturing a surface of the polymer.
 7. The method of claim 6 wherein thetexturing comprises attaching a layer of stiff material to the polymerand relaxing the polymer and the stiff material.
 8. The method of claim1 wherein the forming one or more electrodes comprises spraying the oneor more electrodes onto the pre-strained polymer using a mask.
 9. Themethod of claim 1 wherein one or more electrodes are patternedphotolithographically on the pre-strained polymer.
 10. The method ofclaim 1 further comprising attaching the solid member to a portion of asecond pre-strained polymer.
 11. The method of claim 1 furthercomprising assembling the polymer and the one or more electrodes in anactuator.
 12. The method of claim 1 wherein pre-straining the polymercomprises attaching a structural elastomer to the pre-strained polymer.13. The method of claim 12 wherein the structural elastomer ispatterned.
 14. The method of claim 1 wherein pre-straining the polymercomprises attaching one or more stiffeners.
 15. The method of claim 1wherein the polymer is pre-strained by a factor in the range of about1.5 times to 50 times the original area.
 16. A method of fabricating atransducer comprising multiple pre-strained polymers, the methodcomprising: (a) pre-straining a first polymer to form a firstpre-strained polymer; (b) forming one or more electrodes on the firstpre-strained polymer; (c) pre-straining a second polymer to form asecond pre-strained polymer; (d) forming one or more electrodes on thesecond pre-strained polymer; and (e) coupling the first pre-strainedpolymer to the second pre-strained polymer.
 17. The method of claim 16wherein pre-straining comprises stretching the polymer and fixing it toa solid member.
 18. The method of claim 16 further comprising spincoating a polymer mixture to produce the polymer.
 19. The method ofclaim 18 further comprising releasing the polymer from a rigid substrateused in the spin coating.
 20. The method of claim 19 wherein releasingthe pre-strained polymer from the rigid substrate comprises one of anetchant or peeling the polymer from the rigid frame.
 21. The method ofclaim 16 further comprising repeating (c) to (e).
 22. The method ofclaim 16 wherein coupling the first pre-strained polymer to the secondpre-strained polymer comprises attaching the first pre-strained polymerto the second pre-strained polymer.
 23. The method of claim 1 whereinthe polymer has a maximum actuation pressure in response to electricityprovided by the one or more electrodes between about 0.05 MPa and about10 MPa.
 24. The method of claim 1 wherein the polymer has a maximumactuation pressure in response to electricity provided by the one ormore electrodes between about 0.3 MPa and about 3 MPa.